With increasing demands for bandwidth (BW) in optical networks, technologies are evolving to transmit more bits per second over optical spectrum. Optical spectrum has been standardized such as in ITU-T Recommendation G.694.1 (June 2002) “Spectral grids for WDM applications: DWDM frequency grid” and ITU-T Recommendation G.698.2 (November 2009) “Amplified multichannel DWDM applications with single channel optical interfaces,” the contents of each are incorporated by reference herein. The optical spectrum can be segmented into transmission windows at different wavelengths such as the C band which is about 1530 to 1565 nm and which corresponds to the gain bandwidth of erbium doped fiber amplifiers (EDFAs). Other transmission windows can include the L band (about 1565 to 1625 nm), the S band (about 1460 to 1530 nm), etc. Conventionally, DWDM networks typically use a fixed bandwidth (e.g., 25, 50, 100, or 200 GHz) centered on the ITU grid for each channel (i.e., wavelength). This can be referred to as a gridded DWDM optical spectrum, i.e. each channel occupies a spot on the grid in an associated transmission window. However with higher number of bits per second (especially beyond 100 Gbps), it is getting increasingly complex to fit the channels within a fixed spectral bandwidth (BW). It requires allocating larger bandwidths to fit those high baud rate signals that cannot respect the fixed grid ITU spectrum anymore. Accordingly, there is a movement towards “gridless” or flexible DWDM spectrum where the slot width of the channels is flexible and/or variable (i.e., the slot width is uncertain before a frequency slot is allocated). Optimal spectrum utilization or spectral mining is another reason to move forwards with flexible spectrum solutions where more channels can be embraced together without having any guard band in between that can potentially occupy the full DWDM spectral bandwidth. For comparisons, in a conventional gridded system, each channel has a predetermined bandwidth, i.e. 25, 50, 100, or 200 GHz, but in flexible spectrum systems, each channel has a variable bandwidth of N GHz, where N can be any amount of bandwidth and can be different for each channel.
Referring to FIG. 1, in an exemplary embodiment, a graph of optical spectrum illustrates an exemplary flexible spectrum system 10. The flexible spectrum system 10 includes four channels 12, 14, 16, 18. The first two channels 12, 14 each occupy 50 GHz of bandwidth with guard bands 20 therebetween. A conventional gridded system using 50 GHz spacing would include each channel on the optical spectrum being similar to the channels 12, 14. The third channel 16 occupies 400 GHz BW and can be, for example, a 2 Tbps signal. The fourth channel 18 is a 4×100 Gbps signal with each of the 100 Gbps signals occupying 37.5 GHz BW for a total of 150 GHz. The channels 16, 18 can be referred to as ‘super’ channels and will be more common as more advanced modulation techniques are utilized to increase the number of bits per second over the optical spectrum. In conventional gridded systems, adding or deleting a channel has minimal impact on existing in-service channels since there are many channels in such gridded systems and adding or deleting a single channel has a manageable impact overall. This capacity change problem (i.e., adding or deleting a channel) is significantly more pronounced in flexible spectrum networks since it is no longer adding one channel among many as in gridded systems, but could be adding or deleting a significant portion of the spectrum. For example, adding or deleting the channel 16, 18 will have significant impacts on the other channels 12, 14 in-service.
Thus, capacity changes with flexible spectrum in an optical transmission line system remain as a strong challenge.